Foreign Currency Bank Account Revaluation - how often is too often??

We run a business that has lots of projects some of which have Euro or USD revenues and to a lesser degree Euro/USD cost of sales.

I need to analyse gross profit on a project by project basis. The project report does this perfectly for me.

The only question is how to get the gross profit as accurate as possible - as it converts foreign currency to GBP. So how often should I revalue my Euro and USD bank account?

Should I do monthly or weekly? What are the pro's and con's of doing it more frequently, or less frequently? For example, will it make the project P&L more accurate? Does it create loads of FX lines that my accountant is going to moan about??